Electrolyte composition and process for employing same



United States Patent 6 I EEEC'I-ROLYTECOMPOSITION. AND PROCESS FOR EMPLOYING'SAME Paul L. Sanford; Anaheim, Cuiif. NoDrawing. Application August. 20,1954, Serial No..451,310

28 Claims. (Cl. 204-58) This application is a: coutinuationin-pa'rr of" my cop'endih'g applications Serial" Nos; 237,361 filed July 18, F95 1", (now'abandoned),and297t985, filed July 9', 19-52.

This invention relates to electrolytes; and is particularly" concerned with a novel composition of matter and with electrolytes-containing such composition and which are'e'speciaH'y usefulfor the production of hard wear and corrosion resistant aluminum oxide film'sor coatings on aluminum and aluminumalloys by electrolytic oxidation of the aluminum and such alloys. The invention is also concerned with a process for production of oxide coatings on aluminum and its alloys employing such electrolytes:

Ihasmu'ch as aluminum in its pure or alloyed state is a comparatively soft metal andextremely. susceptible to corrosive action, an attempt has. Been made in the ai'rcra'ftand' other industries to give the metal a protective: plating, or hard coat? The hardest and most'ree si'st'ant coating to wear and to" thecorrosive action of atmospheres containing salt, smoke and corrosive acids is the oxide of aluminum corresponding to corundum AlzQsya Byprocesses. already known to. the. industry this. coating. in limited thickness may. be applied to pure aluminum and someof. its alloys. However, some of the most important aluminum alloys used in the aircraft and other industries have resisted all previously known processes for the successful application of a hard coating because of: the higher." percentages? of alloying. elements, e. g.-,- copperand. silicon, present in these alloys.

As is well? known in this art; the anodic oxidation of aluminum to create oxide films in acid electrolytes,- such as chromic acid, sulphuric acid, or oxalic acid now generally employed, is limited to'the'production of thin films of oxide: Such films may not practicallyhem-excess of about .Olll'" in thickness; The d'enseness of these films i'salso ofmoderate nature; being porous especially when approachingih thickness the dimension of .001. An additional limitation lies in the fact that the processes of theprior art are limited in their application to certain only of the" aluminum alloys: Thus it has heen reported that alloys containing zincor high percentages of copper and silicon willnot produce dense coatings of uniform thickness by such prior art processes;

The process. of my invention employing. the electrolyte hereof, is capable of making thicker, denser and. harder oxide coatings on'a'lumi'num and aluminus alloys than prior art processes known'tome, andalso produces thick, dense, hard oxide coatings. on: aluminum alloys. which prior art. processes.- are' incapable of coating: Improved bonding of the oxide coating to the metal is also obtained, and the coating is deposited more rapidly than heretofore.

ohjection ofl thiss invention: is the provision; of a novel composition: ofi matter-of: particular: use' in electro- Another object is? the: provision of-: a: process for the preparation: of such aiccmposition. 1

Another" object of this invention: isto. provide novel 21' electrolytes, particularly aqueous acidic: solutions for use. in electrolytic oxidation. of aluminum and. its alloys. to produce a protective. oxide coating; thereon.

Still. another aim; of the invention is. to afford pro cedure for applying a tough, hard wear resistantroxidb coating on aluminum and its al-loysby electrolytic oxidation employing the foregoing aqueous acidic solutionsas electrolytes.

Another object is to: produce such: coatings. on; aid.

minum: alloys containing substantial: amounts or copper: I

and/or silicon. A still further object: is to atford a: process providing a hard coating on: aluminum. and its: alloys,. which are highly resistant to the corrosive action of atmospheres containing;isalt, .smoke and acids.

Other. objects: and. advantages 1 of. the: invention. will be apparent from the.- foll'owing 'descriptiomof my'inventiom I have discovered a. novel; composition in the f'onm' of an. aqueous: extractroh brown or low gradejco'al, lig= nite or peat", which: canberobtainedt by extracting brown on low grade coal; 1ignite orpea'ti obtainedfrom'various localities; with water. The: extraction. is. particularly made more rapid and yield is increased by extraction. at elevated. temperature, preferably theatmospheric boiling point. of the. mixture or moreelevated ternperature, e; g. up to- 290F. or highenandmost desirably by extrae tion at elevated temperature under pressure fora periodsufiicient. t'o produce an" aqueous acid solution. found peat" to be partcularly' suitable: for this purpose and thisisthe preferred rawmaterial. The peat usedin practice of' my invention mayb'e derived from variouslocations in the U'nited'States;lfor example; from-Georgia, Florida, Californit or Michigan: 1 In practice, the starting-material, e. g., ligni'te or'Georgia peat isfirst mixed with water ina proportion ofsay 'one part by weight of theground, flaked or fibrous i'ignite', peat or" brown or low grade coal with, for example, six parts of water. These proportions may vary, however. This aqueous mixture is then fed to an" autoclave where high' extraction: temperatures are desired for practicai reasons and Wherein the mixture is cooked at autogenous; pressure and at temperature above its normal boiling point (i e:, the boiling' point of the mixture at atmos phericpressure)". In this respect I have found that satisfactory results' according to the invention are obtaihe d by cooking the brown coal, peat or lignite' at tempera"- tures' between-about 250 F1 and about 350* F. and? at pressures betweenabout 1'0 and about 1'40' p: s. i'. gage for from about 6 to say 50'hours or more, the time re quired to-process theorganic st'arting'material'heing less as the" temperature and pressure are increased; While the above describedhighpressure and high temperature method for producing the extract is preferred,

I cant also: obtain such. extract; by refluxing; the: aqueous mixture of. brown coa-L. lignite or? peat at atmospheric pressure over an extended period of say 24 hours.

After. an? extraction the undi'ssolvedi residue: may be separated. from the extract. In accordance: with. the: invention add the aqueous ext'ractofzbrownrcoal, peat. or lignite to sulphuric, chro micor oxalic acid; on mixtures thereof. The latteracidi: are considered equivalents for the anodic oxidation 'o fi aluminum and its. alloys, and. are. termed: electroarrtitiih. ing. acids herein. Other acids. in addition: tor sulphuric; oxalic. and; chromic acids have been. suggested? by: the; prior art for the: anodic oxidation ofi aluminum;v and those" skilled; in the; art: will. understand: the naturewand type of. such; acids: contemplated herein; it have obtained denser; harder: and: thicker oxide: coatings: employing: my

extract. according-f to the: invention than". heretofore oh tainabl'e; using; aqueous: acid: solutionstwithoutmy ere- I have tract additive. Also the aqueous extract-containing electrolyte of my invention has the important advantage of permitting formation of protective oxide coatings on a wide variety of aluminum alloys, particularly high silicon or copper alloys of aluminum containing, for example, as much at 4% copper or more, when employed in the electroyltic oxidation process, whereas commercial oxide coatings on such alloys have not been possible in the prior art.

In preparing the electrolyte I may employ any of the acids usually employed in making up the electrolyte for electrolytic oxidation of aluminum such as sulphuric acid, chromic acid, or oxalic acid or mixture thereof. Thus, I may use up to say about 70% H2804 in the electrolyte, but I prefer to use dilute H2504 in an amount corresponding to a range of from about 1 part to 20 parts by volume of concentrated (100%) sulphuric acid dissolved in 100 parts by volume of water. For example, i may employ an amount of H280; in the electrolyte corresponding to from about 5 to 7% by volume of 66 Baume' sulphuric acid per 100 parts by volume of water. I generally add to this bath from 1 to 6 parts, preferably about 3 to about 6 parts, by volume of the aqueous extract of brown coal, lignite or peat per 100 parts by volume of water in the electro-anodizing acid solution. it is understood, however, that these proportions are not critical and may be varied.

The electrolytic oxidation of aluminum and its alloys to produce an oxide coating thereon is carried out according to the invention by cleaning the aluminum or aluminum alloy article to be coated and making it the anode of an electrolytic cell which may be in the form of a stainless steel case which acts as the cathode, the cell containing as electrolyte an aqueous electro-anodizing acid solution including my extract. Sufiicient voltage and current density is applied to the cell for a period of time varying from 1 to 120 minutes depending on the thickness and hardness of coat desired. The coated aluminum or aluminum alloy article is then removed from the electrolyte, rinsed in clear water and drain dried.

I may operate my process at the relatively low temperatures conventionally used in oxidation of the aluminum and aluminum alloys, using the conventional electro-anodizing acids as employed in the prior art as described above, and obtain the previously noted improve ments by the addition thereto of the extracts described above. I may employ temperatures in the range of about 10 F. to about 70 F., but prefer to use as low temperatures as convenient, for example, about 20 F. to about 30 F.

The following are examples illustrating preparation of my extract and novel electrolytes containing such extract for the formation of oxide coatings on aluminum and aluminum alloys, and also illustrating the application of such electrolytes in my electrolytic oxidation process.

Example 1 An aqueous extract of the preferred starting material, peat, for use in the invention, can be produced as follows:

One part by volume of Georgia peat ground and mixed is added to 5 parts of water, and the resulting mixture is cooked at 20 lbs. gage pressure for 48 hours. The resulting material is cooled and allowed to settle. The liquid is decanted from the insoluble residue and forms the peat extract additive which is incorporated in the electrolyte according to the invention. This extract is in the form of an aqueous solution of organic acids of a complex nature, and ing properties: the aqueous extract has a pH of from about 4 to about 6, and may contain as little as 2% of dissolved or dispersed solids, depending upon the amount of dilution of the material. The extract upon evaporation to dryness forms a dark brown, glossy residue which is amorphous and has a total nitrogen content of about is characterized by the follow- 4.5 to 5%, for example, 4.8% as determined by the Kjeldahl method. The extract solids are essentially water soluble and form a clear dark brown solution or dispersion. The aqueous extract is preferably kept refrigerated or a fungicidal material such as 2% by volume of 66 Baum H2504 is added.

Example 2 One part by weight of ground peat is mixed with six parts of water and the mixture is fed to an autoclave. The mixture is cooked therein at about 290 F. and at autogenous pressure for six hours. The liquid is decanted and forms an acidic aqueous extract containing complex organic acids, and having a pH of about 4.5 to about 5.5.

Example 3 A .002 inch tough hard coating of oxide of aluminum can be produced on the high copper alloy of aluminum designated 24S and containing about 4.5% copper, by applying a 44 to 48 ampere per square foot current density for 20 minutes to a part formed from such alloy and submerged as the anode in an electrolyte composed of 1.25% by volume of the aqueous extract of Example 2 added to 12% by volume of 100% sulphuric acid in Water solution, the temperature of the electrolyte being maintained between about 20 F. and about 30 F. The coating of oxide of aluminum thus formed will resist 40,000 revolutions of Taber abraser #17 wheels, a standard abrasion testing device, without noticeable effect on the coating. A conventional chromic acid anodize coating on 245 alloy obtained employing an electrolyte not containing the aqueous extract of the invention, fails at 100 revolutions in a similar test, and a conventional hard chrome electroplate coating of .0015" thickness on 248 alloy similarly tested fails at .1000 revo lutions, failure being considered any visual destruction of the coating.

Example 4 In using lignite extract, the extract is formed by the process of Example 1 or 2, substituting ground lignite for the peat and employing the extract in the manner set forth in Example 3.

Example 5 A hard, tough coating of oxide of aluminum is formed on a part composed of a high copper, silicon alloy of aluminum designated as 145, and containing about 4.4% copper and about 0.8% silicon, by subjecting the clean alloy part to 10 volts of direct current for one minute and then to volts of direct current for 20 minutes while the part is immersed as anode in an agitated bath in the form of an aqueous solution consisting of 10% by weight of sulphuric acid and 3% by volume of such solution, of the aqueous extract of Example 1 or Example 2.

I have found that my process is applicable to aluminum and to a wide variety of aluminum alloys containing copper, silicon and other metals such as zinc and nickel. The following are examples of such alloys in addition to 145 and 24S noted above:

Alloy Designation Composition 92.7% At, 1% Si, 3% Mn. 92.5% Al, 4% on, 2% Ni, 1.5% Mn. A1, 1.6% Cu, 2.5% Mn, 5.6% Zn, 0.3% Cr.

The omission of the extract from the electrolyte bath and employment only of H2804 or other electro-anodizing acid does not produce coatings of comparable hardness, density and thickness as'is obtained by-the addition of the foregoing extract to such baths. By employing the aqueous extract'as an additive to the anodizing electrolyte bath, I may obtain oxide coating thicknesses up to and above .002 inch.

While I have described a particular embodiment of tions thereof may be made within the spirit of the invention as set forth'in the appended claims.

I claim:

1. A novel composition of matter: an acidic extract of a substance chosen from the group consisting of low grade coal, lignite and peat, said extract being obtained by cooking a mixture of said substance with water at a temperature of from about the boiling point of said mix ture at atmospheric pressure to about 350 F.

2. A composition of matter as set forth in claim 1, and wherein said cooking temperature is from about the boiling point of the mixture at atmospheric pressure to about 290 F.

3. A composition of matter as set forth in claim 2, and wherein said substance is peat.

4. A composition of matter as set forth in claim 1, and wherein said cooking is carried out at superatmospheric pressure.

5. A composition of matter as set forth in claim 1, and wherein said substance is peat.

6. A composition of matter as set forth in claim 5, and wherein said cooking is had for a period of at least 48 hours.

7. An electrolyte composition of matter consisting essentially of an aqueous solution of an electro-anodizing acid and an acidic extract of a substance chosen from the group consisting of low grade coal, lignite, and peat, said extract being obtained by cooking a mixture of said substance with water at a temperature of from about the boiling point of said mixture at atmospheric pressure to about 350 F.

8. A composition of matter as defined in claim 7, and wherein said cooking temperature is from about the boiling point of said mixture at atmospheric pressure to about 290 F.

9. A composition of matter as defined in claim 8, and wherein said electro-anodizing acid is sulfuric acid.

10. A composition of matter as defined in claim 8, and wherein said substance is peat.

11. A composition of matter as defined in claim 10, and wherein said electro-anodizing acid is sulfuric acid.

12. A composition of matter as defined in claim 7, and wherein said cooking is had at superatmospheric pressure.

13. A composition of matter as defined in claim 12, and wherein said electro-anodizing acid is sulfuric acid.

14. A composition of matter as set forth in claim 12, and wherein said substance is peat.

15. A composition of matter as set forth in claim 14, wherein said electro-anodizing acid is sulfuric acid, and said cooking is had for a period of at least 48 hours.

16. A process ofcoating aluminum and aluminum alloy articles with a hard and tough coating of oxide of alu minum which comprises passing an electric current through an electrolytic cell with said article forming the anode, and containing an electrolyte consisting essentially of an aqueous solution of an electro-anodizing acid and an acidic extract of a substance chosen from the group consisting of low grade coal, ,lignite and peat, said extract being obtained by cooking a mixture of said substance with water at a temperature of from about the boiling point of said mixture at atmospheric pressure to about 350 F.

17. A process as set forth in claim 16, and wherein the cooking temperature is from about the boiling point of said mixture at atmospheric pressure to about 290 F.

18. A process as set forth in c1aim17, and wherein said electro-anodizing acid is sulfuric acid.

19. A process as set forth in claim 17, and wherein said substance is peat. v

20. A process as set forfl'r in claim 19, and wherein said electro-anodizing acid is sulfuric acid.

21. A process as set forth in claim 16, and wherein said electro-anodizing acid is sulfuric acid and said cooking is had at superatmospheric pressure.

22. A process as set forth in claim 21, wherein said substance is peat.

23. A process as set forth in claim 22, and wherein said cooking is had for a period of at least 48 hours.

24. A composition of matter as defined in claim 7, and

wherein said substance is peat.

25. A composition of matter as defined in claim 7, and wherein said electro-anodizing-acid is sulfuric acid and said substance is peat.

26. A composition of matter as defined in claim 7, and wherein said electro-anodizing acid is sulfuric acid, said substance is peat, and said cooking is had at superatmospheric pressure. 1

27. A process as set forth in claim 16, and wherein said substance is peat.

28. A process as set forth in claim 27, and wherein said electro-anodizing acidis sulfuric acid.

References Cited in the file of this patent UNITED STATES PATENTS 2,262,967 Schenk Nov. 18, 1941 2,692,851 Burrows Oct. 26, 1954 2,692,853 Gamble Oct. 26, 1954 

1. A NOVEL COMPOSITION OF MATTER: AN ACIDIC EXTRACT OF A SUBSTANCE CHOSEN FROM THE GROUP CONSISTING OF LOW GRADE COAL, LIGNITE AND PEAT, SAID EXTRACT BEING OBTAINED BY COOKING A MIXTURE OF SAID SUBSTANCE WITH WATER AT A TEMPERTURE OF FROM ABOUT THE BOILING POINT OF SAID MIXTURE AT ATMOSPHERIC PRESSURE TO ABOUT 350* F.
 16. A PROCESS OF COATING ALUMINUM AND ALUMINUM ALLOY ARTICLES WITH A HARD AND TOUGH COATING OF OXIDE OF ALUMINUM WHICH COMPRISES PASSING AN ELECTRIC CURRENT THROUGH AN ELECTROLYTIC CELL WITH SAID ARTICLE FORMING THE ANODE, AND CONTAINING AN ELECTROLYTE CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OF AN ELECTRO-ANODIZING ACID AND AN ACIDIC EXTRACT OF A SUBSTANCE CHOSEN FROM THE GROUP CONSISTING OF LOW GRADE COAL, LIGNITE AND PEAT, SAID EXTRACT BEING OBTAINED BY COOKING A MIXTURE OF SAID SUBSTANCE WITH WATER AT A TEMPERATURE OF FROM ABOUT THE BOILING POINT OF SAID MIXTURE AT ATMOSPHERIC PRESSURE TO ABOUT 350* F. 